I would like to trigger an input based on the beam between an IR LED and Photodiode being broken. The attached image is a circuit I found to do this with a Phototransistor. If I want to do the same but with a photodiode instead, can I simply replace the PT with a reverse bias PD?
The reason I want to use the PD is that I already have some BPV10NF photodiodes in my parts bin and from what I read a PD is more sensitive than a PT which might be beneficial. I like the look of the attached schematic as my understanding is that the SCR acts as a type of latch - holding the input in the triggered state.
Am I correct in thinking i can simply substitute the PT for a reversed PD or is there more to it that that?
What is the distance between light source and photo transistor, and what detection speed.
And is there any other light (artificial/daylight) involved.
Unmodulated IR (BPV10NF) might only work in the centimeter range.
Leo..
Probably not, because the dude who answered it has drawn the diode the wrong way round,
assuming you want a positive voltage on the output of the opamp.
Leo..
Ideally I'd like to avoid the opamp route as I have the transistors and SCR in my parts bin already. The latching effect of the SCR is also desirable.
This is for a (low power) ballistic chronograph. The LED and PD will be 1-2cm apart, enclosed in a tube. The projectile will be travelling at 300-1000 FPS so the change in output from the PD will be very brief.
The circuit I have referenced is known to work for this use-case. I found it used in a similar project based on NodeMCU rather than Arduino.
I'm asking what other changes (if any) will be required if I want to use a PD rather than PT and looking to confirm the PD should be used in reverse bias - so the cathode connecting to the 2N2222/resistor junction and anode to GND? Based on reading I think the PD is more sensitive and responds more quickly when used this way.
Wawa:
Probably not, because the dude who answered it has drawn the diode the wrong way round,
assuming you want a positive voltage on the output of the opamp.
Leo..
You see this is what confuses me. I read some references and think I understand how the circuit should work, then I read another example and it seems to contradict and I don't know which is correct.
Regards the example I found above, my understanding is:
PT allows current to flow through the resistor to GND when it receives light.
The junction between the resistor and PT (2N2222 Base) is a NOT gate.
When the PT stops receiving light, current cannot flow to ground via the PT which has the effect of putting the 2N2222 base high.
The 2N2222 amplifies this to a level that will trigger the SCR which in turn pulls the IO pin to low.
It seems to me that if I install a PD in reverse bias in place of the PT the effect will be the same. When the PD receives light it's resistance to current flow reduces and current flows through it to ground. When the light beam is broken, current will not flow and the 2N2222 triggers the SCR.
Appreciate I will need to tinker with the various resistors to influence the output/trigger point.
Wawa:
Projects like this have been done before.
Google "ballistic chronograph site:arduino.cc".
Leo..
You're missing the point. I'm not asking about how to create this type of project. I'm asking about directly substituting a PT for a PD and how any given circuit may need to change to accommodate that.
substituting a PT for a PD and how any given circuit may need to change to accommodate that
That would depend on the light levels in your application, on the specific PT chosen, the specific PD, and the given circuit. Consult datasheets for the chosen devices.
can I simply replace the PT with a reverse bias PD?
Perhaps, but most likely not. Phototransistors have gain.
If you have to ask these really basic questions, why try to modify a given circuit, rather than build one that is known to work?
What speed of response do you need? Worth checking the response time of any SCR, they aren't necessarily
brilliant, and phototransistor is probably ruled out, photodiode inherently much faster especially
if reverse biased.
A cheap fast comparator like a LM339 sounds a much more flexible component for this kind of circuit, in fact
its exactly the kind of thing comparators are meant to do.
An SCR could be driven by the comparator output (don't forget they are open-collector outputs),
or an SR flipflop could be the latching component.
